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History as the Flux of Populations
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Citation: Moreno-Estrada A, Gravel S, Zakharia F, McCauley JL, Byrnes JK, et al. (2013) Reconstructing the Population Genetic History of the Caribbean. PLoS Genet 9(11): e1003925. doi:10.1371/journal.pgen.1003925

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Father of Lies

Father of Lies

More often than not the discipline of history seems to swing between the true and trivial (or perhaps more precisely, picayune), and grand narratives which emphasize a nearly fictionalized story. In some ways this is not entirely a problem. When teaching young children the history of the United States a punctilious adherence to fact is essential, but, one can not deny that the selection of topicality can sway and shade the direction of the lessons learned. But far too often this ideological element of the historical narrative determines the central focus, rather than floating along the margins. With erudite command of detail historical scholars can, if they so choose, engage in a game of ideological sophistry, cultural flattery, and underhanded polemic. Both Howard Zinn and David Barton were and are players at this game. But there are still those who engage in the Sisyphean task of perceiving the world as it is, not as we would wish it to be, through the dark glass. Such a colossal enterprise, to ascertain the objective character of an exceedingly complex phenomenon, requires every tool at hand. Historians have traditionally been hunters of musty texts in neglected libraries, but they have on many an occasion received auxiliary data from scholars working in more material domains, such as archaeologists and engineers. Today you must add geneticists to the growing brigade of scholars attempting to excavate the past.

It is known

It is known

In truth the power of genetics is most evident and necessary in areas where history is silent, before written records can build a narrative skeleton in which we can play. Using both modern and ancient DNA samples the geneticists, working with archaeologists, can still make vague inferences where before there was only darkness. But illumination can be had even in time periods when historical records are quite good. Though the public understands evolution to transpire over eons, basic population genetic processes occur over a matter of generations, and so can give us fresh insight into dynamics which played out quite recently in time. A new paper in PLoS GENETICS, Reconstructing the Population Genetic History of the Caribbean, does just this. Obviously we already have a history of the Caribbean. As every schoolboy knows it began in 1492, and proceeded across the centuries as a palimpsest of European colonial powers, and later independent nations, rose and fell. But history is more than just wars, international congresses, and once-in-a-generation discoveries. It is the ebbing and flowing of peoples themselves in their aggregate masses. Conventional textual narratives and coarse archaeological inferences can get us rather far. See Charles C. Mann’s magisterial 1493 for an example. But historical population genetics goes a step further, as it attempts to infer demography through patterns of variation in genes, the most elemental instrumental variable for tracing demographic patterns one might imagine.

journal.pgen.1003925.g004 What the above paper does is reiterate, emphasize, and clarify, particular population genetic demographic events which have been suspected. First, the Amerindian populations were not static creatures in equilibrium with nature, but dynamic. There is clear evidence in these results that some groups migrated from South America to Central America, and especially the Caribbean. This is not unreasonable a priori, but far too often our stylized models presume the Amerindian population as a homogeneous, uniform, almost ahistorical substrate upon which European agency and African tragedy can unfold. But on the contrary, the peoples of the New World had their own history, oral as it may be. As you can see the Maya, one of the most iconic of Amerindian peoples, seem to exhibit some southern affinities, perhaps the result of an ancient “back migration.” If the Old World is any guide there may have been many forward and back migrations.

journal.pgen.1003925.g004 This ancient legacy is evident in the admixed populations, the Mestizos, Zambos, and Mulattos of the Greater Caribbean region. Looking in particular at the Puerto Rican and Dominican populations you see low, but significant, levels of admixture from specific native groups. One the one hand you may not be surprised, but it must be stated that before the genetic evidence there was much skepticism as to whether any Amerindian genetic heritage persisted in the populations of the Caribbean. A particular style of cultural/humanistic scholar intuited that perhaps an emphasis on indigenous ancestry was a mechanism for people of some African ancestry to deflect attention away from this aspect of their heritage because of the fraught history of slavery. Though the internal logic here seems reasonable, the empirical evidence makes it clear that the legacy of the Amerindians does persist in these islands, among these peoples. Their motive may have been unpalatable, but their argument was right.

Who were these Amerindian people? And how did they become integrated with the synthetic populations which came to dominate these islands? This is where textual history and genetics operate in a complimentary fashion. Both history and ethnography document mass population collapse in concert with an androcide of the Amerindians. By this, I mean that European males took Amerindian women as concubines, and engaged in de facto polygyny in the New World. Hernan Cortes, conqueror of the Aztecs, illustrates his phenomenon, as he had an illegitimate son, Martin Cortes, with his native translator, and later on a legitimate son, another Martin Cortes, with a Spanish noblewoman. This pattern of sexual liberty and license was common in the early years, and has been extensively documented by historians. It is a reason many anthropologists give for the relatively low rates of legitimacy in much of Latin America. And of course what applied to Amerindian females also applied to African females. What the genetics makes clear is that this asymmetric pattern of cultural power relations was demographically very significant. Populations with a near total lack of Amerindian and African Y chromosomal lineages, passed from father to son, may still have high levels of non-European mtDNA, passed from mother to daughter. In this study they also looked at the X chromosome, which spends 2/3 of its time in females, and did find an enrichment of Amerindian ancestry there as well.

Tract But they didn’t just focus on the nature of admixture today, they inferred its history. The technique is rooted in basic concepts in genetics. When you have chromosomes come together from parents in a child, those are distinct and identical in nature to segments of ancestry one might find in parents. But genetic recombination in the next generation shuffles the segments, so that parental elements become mixed together on the same segment. When parents are from different geographic populations you see alternative segments of “ancestry tracts.” For example a chromosomal segment with alternative regions of European, Amerindian, and African, ancestry. Because there are only 20-30 recombination events per generation per individual the distribution of the length of these tracts is a function of the length of time since admixture. The early years after admixture will be characterized by long blocks of ancestry from one population, alternated with another. As time passes the segments will get smaller, and alternate much more rapidly. What the authors found was that indeed Amerindian segments exhibited the latter pattern, while European and African segments were more diverse in their distribution. The distinction was strongest in the Caribbean populations, but was evident elsewhere. The explanation is the one above. The early years of Iberian settlement were characterized by de facto polygyny and decimation of male Amerindians through enslavement (though there was population collapse more generally due to disease). Amerindian ancestry came in one singular pulse, and slowly dissipated and distributed itself through the population.

Finally, the results here also yield the finding that Latin American European ancestry seems to have diverged from its parent source. A detailed exploration of the technical issues can be found at the Haldane’s Sieve weblog, but I will say I am convinced that the authors have made a good, if not definitive, case for the proposition that the Latin American ancestral component is one which has diverged significantly. Again, the reason was listed above: de facto polygyny. This drives down the effective population, increases the drift, and skews the allele frequency distribution rapidly away from the source population. If this is a true result it shows us the possibilities for how new populations can arise through fission and rapid expansion. In particular, they may be male mediated. For this period, from 1500-1900, we have extensive documentation to corroborate the broad inferences made. But not so for many regions deep into the past. What these sorts of papers illustrate is the fine-grained power of genetics in shedding light on topics and issues which might otherwise have remained off limits. In particular genetics taps into some of the most primal activities of humankind, those that lead to procreation.

Citation: Moreno-Estrada A, Gravel S, Zakharia F, McCauley JL, Byrnes JK, et al. (2013) Reconstructing the Population Genetic History of the Caribbean. PLoS Genet 9(11): e1003925. doi:10.1371/journal.pgen.1003925

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  1. * This was an incredibly intense demographic shift. A quote from the paper illustrates just how intense the demographic shift was at some points in time:

    “The best-fit model for Colombians and Hondurans involves admixture between Native Americans and Europeans starting 14 generations ago, followed by a second pulse of European ancestry starting 12 and 5 generations ago, respectively. Of note is that between the first and second pulse of migration in Colombians, the proportion of European ancestry increased from 12.5% to 75% in two generations, implying that the European segments in today’s Colombians date back to European gene flow happening in a short period of time; thus, tracing their ancestry to a smaller number of European founders compared to other Latino populations.”

    To get to 75% European ancestry in two generations from 0% would imply that 100% of fathers were European (and 0% of mothers were). Since the starting point was 12.5%, the percentage of fathers who were European was less than 100% – my back of napkin estimate would be that 66% of fathers would have to be European to see that shift in two generations (assuming that 0% of mothers were any % European, which due to admixture would be an underestimate, so the reality might be closer to 50%-60% in those two generations despite the number of Europeans being far less than the number of indigeneous men).

    * What precisely did “androcide” mean in this context?

    It would be nice to know what “androcide” means in a quantitative sense. I searched the paper, in vain (perhaps it was in the supplemental materials), for some simple metrics that compare the relative degree of indigeneous male and indigenous female contributions to the modern gene pools (e.g. the percentage of Y-DNA haplogroups and mtDNA haplogroups in the modern population that are indigenous in origin). For example, a 2008 paper by Hammer et al. summarized this with an estimated breeding sex ratio (i.e., the effective size of females to males) that ranged to as high as 12.5 for the French Basque, but I can’t work out a similar metric for this data myself although surely this study had sufficient raw data to calculate these metrics.

    It is also hard, for people like myself who didn’t learn this history, to a sense of what the lives of indigenous men deprived of an opportunity to have children were like in that transition period. Were they literally killed, isolated into slave labor gangs, exiled to be hunters or bandits in the wilderness, or what? Likewise, it would be interesting to have some snippet of what the indigeneous women felt about the situation. Was having a half-European child something prestigious that they were competing with each other to have or did it feel like humiliating servitude?

    * How does the Caribbean experience compare to other examples?

    One of the reasons to care about how the Caribbean process played out is to understand what might have happened in prehistory where we don’t have historical documents to frame the genetic data and confirm the interpretation given to an analysis of it.

    It would also be interesting to compare Caribbean situation compares to other historical and prehistoric cases of admixture. Presumably, this is a situation where the European admixture was particularly dominant given the immense disparities of power and first contact disease exposure issues in the immediate post-1492 era. One would imagine that two groups less starkly outmatched would experience a muted version of the same process.

    It seems as it the pattern in North America involved more replacement and less admixture.

    The encounters of first wave Neolithic farmers and European foragers before the Chalcolithic era seem to have involved little early admixture, quite unlike the Caribbean situation. Could this have been because early farmers weren’t dominant enough to overwhelming early foragers when their own farming techniques were quite primitive and they lacked metal weapons to give them a military advantage?

    The Yaoyi-Jomon admixture in Japan seemed to involve much of the indigenous Jomon component (both men and women) remaining in the gene pool, despite near total destruction of the Jomon language and culture. The ethnogenesis of Madagascar, likewise seems to have involved fairly equal contributions of African and Indonesian genetic components, less stark differences in male and female contributions, and an overwhelmingly complete elimination of African linguistic and cultural contributions.

    Are there populations whose prehistoric admixtures look demographically similar to the Caribbean?

    Obviously, nobody expects a single blog post to answer these questions. But, these are the questions that start running through my head as I read your post.

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